Scientists are building walls of willow trees in the Southern French Alps gullies to trap dirt that blocks hydroelectric dams downstream.
Sediment in rainwater washes off the steep valley slopes in the Francon marly basin into gullies. These smaller waterways then feed larger rivers. When the water reaches the slower-moving reservoirs above dams, the sediment drops out and settles behind the barrage.
This can cause big problems as dams are only engineered to withstand a set about of force pressing against them. If this is not removed periodically, the dam can eventually burst or overflow. So the muddier the river, the faster the build up. Every 15 years, energy companies in must pay millions of euros to scoop out the mounds of dirt.
Freddy Rey and colleagues are adapting ancient bioengineering systems, high up in the mountains, to prevent the amount of sediment reaching the dam. Willow cuttings are buried 30cm within wooden walls that stretch across the channels. These are placed roughly every three meters up the gully like a staircase.
Similar to a sieve, the willows allow water to flow through but trap the sediment back. Within a year, these structures can trap up to 10cm of sediment. Over time, this increases as the willows become more established and more sediment can be kept back. After 20 to 30 years, the vegetation will have complete cover and sediment erosion is almost completely stopped.
In with the new
Rey and his colleagues figured that instead of trying to prevent erosion on the banks of the gorges, it was cheaper to plant the walls actually in the gully base. “The sediment is eroded off the slopes but is instead trapped in the gully floor,” says Rey. In the long run, the bioengineering works out more cost effective.“It costs 15 Euros per m3 of sediment to scoop dirt out of the dam and this has to happen every 15 years,” says Rey. “The willow walls however have a one off cost of four Euros per m3. Once they are in – they don’t need to be replaced.”
Looking back in time
The work presented at the EGU only started in 2002 and so in early days. The team needed to see how reliable these structures were and if they hold out in the long term and especially during intense rainfall. They looked for evidence in similar structures, using Austrian black pine plantations, that were used 136 years ago to stop soil erosion caused by agriculture.
“If you dig down into the sediment of these old walls – you can see the original gully shape and the result of 150 years of action,” says Rey. “We know there has been heavy rainfall during this time and so know the structure have stayed in place.”
Location, location, location
The walls in this research were placed in gullies in the Francon catchment, spanning an area of around 50,000ha. However, the structures must be strategically placed. If conditions are too dry, for example, the cuttings cannot get established into saplings and sediment trapping is ineffective.
‘‘It is really important to find the right places,” says Rey. “Even in France, if the aspect is to the south we can have problems.”
Other experiments are using cuttings from different species such as Popular (Populus nigra) and rooted plants such as Seabuckthorn (Hippophac spp.) to see if the amount of dirt trapping can be increased.
by Becky Summers, City University, London